Can Stress-Induced CAM Provide for Performing the Developmental Program in Mesembryanthemum crystallinum Plants under Long-Term Salinity?

The development of CAM-type photosynthesis is one of the adaptation mechanisms to severe water deficit. It provides plants with carbon dioxide and permits efficient water spending under extreme environments. In common ice plants, a complete switch from C₃ to CAM photosynthesis was observed on the seventh day of salinity (0.5 M NaCl). The indices characterizing this switch were: (1) induction of phosphoenolpyruvate carboxylase; (2) diurnal changes in the organic acid content, which are characteristic of CAM plants, and (3) suppression of transpiration during the daytime. A decrease in the osmotic potential () of the leaf sap, which occurred on the second day of salinity, preceded these changes. After long-term salinity stress (four–five weeks), attained extremely low values (–4.67 MPa), which made possible the water uptake by the root system. The restoration of the balance between cell compartments resulted from the accumulation of compatible solutes in the cytoplasm, proline primarily, which possesses osmoregulatory and stress-protective properties. This means that a complex of adaptive mechanisms is required for the realization of the common ice developmental program under salinity. These mechanisms maintained plant capacity to uptake water and permitted its efficient utilization. They triggered the development of stress-induced CAM-type photosynthesis, maintained the low osmotic potential in the cell sap, regulated the composition of macromolecules in the cell microenvironment, provided for water storage in tissues, and reduced the time of plant development. A comparison between the time-courses of CAM development and a decrease in the transpiration rate permitted us to suggest that a combination of low and CO₂ in the leaf cells could serve as a signal for the induction of CAM-dependent gene expression in terrestrial plants.     

Chloroplast acclimation to low osmotic potential

Photosynthetic potential of isolated chloroplasts was investigated during in situ water deficits. An eight day stress cycle imposed on spinach plants reduced leaf _w by 0.57MPa, and leaf by 0.50MPa, resulting in partial turgor maintenance during the stress cycle. Pressure/volume curves confirmed the occurrence of osmotic adjustment. Leaf depression was associated with an altered response of chloroplasts to low in vitro. Optimum reaction medium for photosynthesis shifted from –1.04 to –1.57MPa, and low was not as inhibitory to photosynthesis of plastids pre-exposed to stress in situ. These data indicate that chloroplasts acclimate to low external in response to leaf water deficits. This response was still evident four days after a stress cycle ended, but was nearly reversed eight days after stress. Repeated stress cycles in situ did not increase the degree of chloroplast acclimation to low in vitro. Fast dehydration of leaves did not induce this apparent chloroplast acclimation.Abbreviations osmotic potential - _w water potential - PEG polyethylene glycol 8000 - MPa megapascals     

Photosynthesis of isolated chloroplasts and protoplasts under osmotic stress

1. Isolated intact spinach chloroplasts respond to changes of the sorbitol concentration of the suspending medium as near-perfect osmometers within a large range of osmotic potentials. Under isotonic conditions (=9–10 bar), their average osmotic volume is 24 m³ and the total volume 36 m³. The osmotic volume can be increased to 63 m³ by lowering the sorbitol concentration until a critical osmotic potential of =4 bar is reached. Below that value chloroplasts rupture. Between 10 bar and 4 bar, volume changes are reversible. 2. Increasing the chloroplast volume above 24 m³ causes inhibition of photosynthesis, with 50% inhibition occurring at an osmotic potential of =5–6 bar. This corresponds to an osmotic volume of 45–55 m³. Depending on the duration of hypotonic treatment, inhibition of photosynthesis is more or less reversible. 3. Between 4 and 10 bar, the chloroplast envelope exhibits a very low permeability for ferricyanide, many metabolites, and soluble stroma proteins. 4. Electron transport is not inhibited by swelling of chloroplasts. Also, the ATP/ADP-ratio remains unchanged. 5. The solute concentration in the chloroplasts appears to be optimal for photosynthesis at 10 bar. Increasing the chloroplast volume causes inhibition of photosynthesis by dilution effects.     

Control of photosynthesis by the carbohydrate level in leaves of the C4 plant Amaranthus edulis L.

Photosynthesis was studied in relation to the carbohydrate status in intact leaves of the C₄ plant Amaranthus edulis. The rate of leaf net CO₂ assimilation, stomatal conductance and intercellular partial pressure of CO₂ remained constant or showed little decline towards the end of an 8-h period of illumination in ambient air (340 bar CO₂, 21% O₂). When sucrose export from the leaf was inhibited by applying a 4-h cold-block treatment (1°C) to the petiole, the rate of photosynthesis rapidly decreased with time. After the removal of the cold block from the petiole, further reduction in photosynthetic rate occurred, and there was no recovery in the subsequent light period. Although stomatal conductance declined with time, intercellular CO₂ partial pressure remained relatively constant, indicating that the inhibition of photosynthesis was not primarily caused by changes in stomatal aperture. Analysis of the leaf carbohydrate status showed a five- to sixfold increase in the soluble sugar fraction (mainly sucrose) in comparison with the untreated controls, whereas the starch content was the same. Leaf osmotic potential increased significantly with the accumulation of soluble sugars upon petiole chilling, and leaf water potential became slightly more negative. After 14 h recovery in the dark, photosynthesis returned to its initial maximum value within 1 h of illumination, and this was associated with a decline in leaf carbohydrate levels overnight. These data show that, in Amaranthus edulis, depression in photosynthesis when translocation is impaired is closely related to the accumulation of soluble sugars (sucrose) in source leaves, indicating feedback control of C₄ photosynthesis. Possible mechanisms by which sucrose accumulation in the leaf may affect the rate of photosynthesis are discussed with regard to the leaf anatomy of C₄ plants.Abbreviations and symbols A net CO₂ assimilation rate - Ci intercellular CO₂ partial pressure - PEP phosphoenolpyruvate - RuBP ribulose-1,5-bisphosphate - water potential - osmotic pressure     

The effect of potassium application on leaf water relations characteristics of field grown barley plants (Hordeum distichum L.)

Leaf water relations characteristics were studied in spring barley fertilized at low (50 kg ha^-1) or high (200 kg ha^-1) levels of potassium applied as KCl. The leaf water relations characteristics were determined by the pressure volume (PV) technique.Seasonal analysis in fully irrigated plants showed that within 2 weeks from leaf emergence the leaf osmotic potential at full turgor ( ¹⁰⁰) decreased from about –0.9 to –1.6 MPa in leaf No 7 (counting the first leaf to emerge as number one) and from about –1.1 to –1.9 MPa in leaf No 8 (the flag leaf) due to solute accumulation. ¹⁰⁰ was 0.05 to 0.10 MPa lower in high K than in low K plants. Thus, an ontogenetically determined accumulation of solutes occurred in the leaves independent of K application. The ratio of leaf weight at full turgor to dry weight (TW/DW) decreased from about 5.5 in leaf No 6 to 4.5 in leaf No 7 and 3.8 in leaf No. 8. The TW/DW ratio was 4 to 10% higher in high K than in low K plants indicating larger leaf cell size in the former. The tissue modulus of elasticity () was increased in high K plants. The main effect of high K application on water relations was an increase in leaf water content and a slight decrease in leaf During drought limited osmotic adjustment and increase in elasticity of the leaf tissue mediated turgor maintenance. These effects were only slightly modified by high potassium application.     

Ecophysiological analysis of woody species in contrasting temperate communities during wet and dry years

This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (g _wv) and low midday leaf water potential (), whereas the wet-mesic community had low A and g _wv and high midday . The mesic and wet-mesic communities had dry year decreases in predawn , g _wv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and g _wv tended to have low midday . This trend was reversed only in the mesic community in the drought year. Despite differences in midday , all three communities had similar midday leaf turgor pressure (_p) in the wet year attributable to lower osmotic potential at zero turgor ( ⁰ ) with increasing site droughtiness. Lower wet year ⁰ in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ⁰ in the wet year often did not have the lowest ¹⁰⁰ possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ¹⁰⁰ but not in ⁰ , via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.     

Influence of intracellular and extracellular tonicities on water permeability in Characean cells

Summary The effect of the concentration of the central vacuolar sap on water permeability previously demonstrated onNitella internode (Tazawa and Kamiya 1966), has been further studied. By using a technique of vacuole perfusion the ionic concentration of the cell sap has been modified independently of its tonicity. Transcellular water permeability has been measured by means of a double-chamber osmometer.When the tonicities of artificial saps were adjusted to that of the natural cell sap, wide variations in the concentration of K⁺, Na⁺, or Ca⁺⁺ in the vacuole did not bring about any change in the magnitude of water permeability. On the other hand, water permeability was strongly influenced by varying the tonicity of the vacuolar medium by addition of mannitol. It increased when the tonicity was lowered from the normal level, while it decreased when tonicity was heightened. Water permeability was also decreased by increase in the tonicity of the external medium.Analysis of the results showed that the specific resistance to water flow across the plasmalemma and the tonoplast in series (the reciprocal of the water permeability k_p) was related to the osmotic pressures of the intracellular ( _i) and the extracellular ( ₀) medium by the empirical formula, l/k_p=0.088 + 0.015 . + 0.0074 ₀. Thus, intra- and extracellular tonicities influence the water permeability of theNitella internode independently of each other. The decrease in water permeability by increase in tonicity of the intra- or extracellular medium may be explained in terms of the effect of these tonicities on hydration of the cell membranes.The water permeability ofLamprothamnium, a brackish water Characeae was only one fourth that ofNitella, a fresh water Characeae. The lower permeability inLamprothamnium may be accounted for in terms of the high tonicities of its cell sap and external medium.     

Acclimation of photosynthesis in Zea mays to low water potentials involves alterations in protoplast volume reduction

Effects of water-stress treatment of Zea mays L. plants on protoplast volume and photosynthesis in leaf slices exposed to solutions of different osmotic potential ( _s) were studied. Decreased photosynthetic capacity in the leaf slices at low tissue _w was associated with dehydration-induced protoplast-volume reduction. Leaf slices from plants exposed to in-situ water deficits exhibited greater photosynthetic capacity and relative protoplast volume at low water potential ( _w) invitro than tissue from control plants.In-situ water stress induced osmotic adjustment of the leaf tissue as determined by pressure/volume analysis. It is concluded that plant acclimation to low leaf _w may involve a reduced degree of cell shrinkage at a given _w. This acclimation would allow for the maintenance of relatively higher photosynthetic capacity at low water protentials.Symbols _s Osmotic potential - _w water potential New Jersey Agricultural Experiment Station Publication No. 12149-6-87     

Combined effects of drought and high temperature on water relations of wheat and sorghum

Drought and high temperature are major factors limiting crop production. The two stresses occur together in many regions, but they usually are investigated separately. This study tested the hypothesis that high temperature interacts with drought to affect water relations, and the effect is greater in heat-sensitive wheat (Triticum aestivum L.) than in sorghum (Sorghum bicolor L. Moench). Wheat and sorghum were grown in soil that was well watered or not watered in controlled chambers at 15/10, 25/20, 35/30 and 40/35 °C day/night. Soil water content (SWC), leaf relative water content (RWC), leaf water potential (), leaf osmotic potential (), leaf turgor potential (P) and osmotic adjustment (OA) were determined at 2-d intervals. All values held nearly constant at all temperatures when soil was well watered but were affected strongly by high temperature when water was withheld. The combined stresses reduced SWC, RWC, Psi and , and unevenly raised P over time, particularly in sorghum. Sorghum also exhibited marked OA at high temperature, which was usually lethal to wheat. High temperature appeared to interact with drought to affect water relations by altering SWC and not by influencing OA. The results demonstrated that crops maintain nearly stable water relations regardless of temperature when moisture is ample, but high temperature strongly affects water relations when water is limiting. Increasing the thermotolerance of wheat might improve its potential to acclimate to both high temperature and drought.     

Cell-wall tension of the inner tissues of the maize coleoptile and its potential contribution to auxin-mediated organ growth

Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ^* of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -^* and the osmotic pressure of the cell sap (_i) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -^* was 6.1–6.5 bar at a _i of 6.7 bar. Both -^* and _i decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.Abbreviations and Symbols _i osmotic pressure of the tissue - ₀ external water potential - ^* water potential at which segment length does not change - IAA indole-3-acetic acid - ITW longitudinal inner tissue walls - OEW outer epidermal wall - P turgor Supported by Deutsche Forschungsgemeinschaft (SFB 206).     

Kinetics of growth and lactic acid production in continuous and batch culture

Summary In a lactic acid fermentation by Streptococcus faecalis, the specific consumption rate of glucose (v) and the specific production rate of lactic acid () were represented by the following simple equations as functions of the specific growth rate (): 1/=(1/) + 1/ = (1/) + By use of data from a batch culture, these two equations were derived from enzyme kinetics of the product inhibition. These equations were successfully applied to the results of batch culture and chemostat culture. In addition, calculation of ATP yield by these equations agreed with the experimental results better than the conventional Leudeking-Piret type equation, which includes two terms associated with growth and not with growth. Correspondence to: H. Ohara     

Osmotic potential and turgor maintenance in Spartina alterniflora Loisel.

Summary The dependence of leaf water potential (), osmotic potential () and turgor pressure (P) on relative water content (RWC) was determined for leaves of tall and short growth forms of Spartina alterniflora Loisel. from a site on Canary Creek marsh in Lewes, Delaware. Tall plants (ca. 1.5 m) occured along a drainage ditch where interstitial water salinity was approximately 20, and short plants (ca. 0.2 m) were 13 m away near a pan and exposed to 80 salinity during the most stressful period. Leaves were collected at dawn and pressure-volume measurements were made as they desiccated in the laboratory. Pressure equilibrium was used to measure , RWC was determined from weight loss and dry weight, was determined from the pressure volume curve, and P was calculated as the difference between and . Physical properties of the bulk leaf tissue that have a role in regulating water balance of the two growth forms were estimated: relative water content of apoplastic water (RWC_a) relative water content at zero turgor (RWC₀), the bulk modulus of elasticity (E), and water capacity (C _w). There were no detectable temporal trends in any of the parameters measured from Nune through September and no significant differences between the two growth forms when compared on the basis of RWC_a, RWC₀, E, and C _w. There was a clear difference between the two growth forms with respect to ; at RWC₀, was-4.5±0.40 MPa for short form plants and-3.3±0.40 MPa for tall form.Turgor pressure of plants in the field (P) was lower in leaves from short form than for the tall form plants with average difference of about 0.4 MPa. In July, P in short form leaves dropped to zero by mid-morning as expected for leaves experiencing water stress.These results show that S. alterniflora is capable of reducing osmotic potential in response to increased salinity and that turgor pressure was lower in short growth form than in tall forms.     

Leaf water relations and anatomy of a tropical rainforest tree species vary with crown position

Summary Leaf water potentials, osmotic properties and structural characteristics were examined in the Australian tropical rainforest tree species, Castanospermum australe. These features were compared for individuals growing in the understorey and canopy of the undisturbed forest and in an open pasture from which the forest had been cleared. Leaf water potentials during the day declined to significantly lower values in the open-grown and canopy trees than in the understorey trees. During most of the day the opengrown tree experienced the lowest water potentials. These differences were paralleled by significant differences in tissue osmotic properties. The tissue osmotic potential at full hydration was lowest in the open-grown tree (-1.80 MPa), intermediate in the canopy trees (-1.38 MPa), and highest in the understorey trees (-0.80 MPa). As a result, the degree to which high and positive turgor pressures were maintained as water potentials declined was highest in the open-grown tree, intermediate in the canopy trees, and lowest in the understorey trees. The differences in tissue osmotic properties between individuals in the three crown positions were paralleled, in turn, by differences in leaf structual characteristics. Relative to leaves of the canopy and open-grown trees, leaves of the understorey trees had significantly larger epidermal cells with thinner cell walls, larger specific leaf areas and turgid weight: dry weight ratios, and a higher proportion of intercellular air space.Abbreviations ₁ Leaf tissue water potential - _min Lowest value of ₁ during the day ( noon) - _{P=0 1} zero turgor - R Relative water content - P Tissue turgor pressure - Tissue osmotic potential - ₀ at full hydration     

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (sleeves) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences ( ) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied ( =±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10^–8 m·s^–1. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10^–7 m·s^–1 MPa^–1 were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.Abbreviations and symbols a _s outer surface of sleeve segment - c concentration of osmotically active solute - j _{v, s} radial volume flow density across sleeve segment - Lp_s hydraulic conductivity of sleeves - Lp_r hydraulic conductivity of intact roots - _N thickness of Nernst diffusion layer - reflection coefficient of root for solute - osmotic value of bulk phase - osmotic coefficient     

Histochemische Untersuchungen über die π-Granula (Reich) der peripheren markhaltigen Nervenfasern

Zusammenfassung Nach dem 4. Lebensjahr sind die nach Reich benannten Protagon () Granula regelmäßig in den Schwannschen Zellen normaler segmentierter Nervenfasern des Menschen vorhanden. Im Senium und bei kachektischen Zuständen der verschiedenen Genese treten sie vermehrt auf. Die -Granula sind an den Nervenfasern vom Hund, Schaf, Kaninchen und Tiger nachweisbar, fehlen aber bei folgenden Wirbeltieren: Rind, Ziege, Schwein, Katze, Ratte, Meerschweinchen, Maus und Frosch. Nach tmserer histochemischen Analyse stellen die -Granula stark chromotrope, saure [relativer isoelektrischer Punkt bei p_H (0,9) 1,5-1,8] Bial-negative Glykolipide dar, die am meisten den Cerebrosiden und Cerebrosidschwefelsäureestern (Sulfatiden) entsprechen; für die Beteiligung von Phospholipiden, Polysacchariden und Proteinen an ihrem Aufbau ergab sich kein sicherer Anhalt. Die Färbung mit essigsaurem Kresylviolett zeigt eine bräunliche Metachromasie der -Granula. Die Bedingungen für eine braune Metachromasie sind bis jetzt noch nicht völlig geklärt. Auch formalininfixierte Markscheiden können sich nach kräftiger Wässerung (Lösung reversibler Formalinbindungen) mit der Feyrterschen Thionin-Einschlußmethode braun färben. Wir führten systematische vergleichende Untersuchungen über die Wirkung verschiedener Extraktionsmittel auf die -Granula von formalinfixierten und unbehandelten Nerven durch; die Einzelheiten sind im Original nachzulesen. Nicht nur an Nervenfasern von Erwachsenen, sondern auch von menschlichen Feten, und einigen Tierarten, die keine -Granula enthalten, ist die savre Phosphatase im perinukleären Zytoplasma der Schwannschen Zellen nachzirweisen.Zum ehrenden Gedenken an meinen Lehrer in Anatomie, Herrn Prof. Dr. med. habil. Kurt Alverdes (Leipzig).     

Does glutathione S-transferase Pi (GST-Pi) a marker protein for cancer?

Glutathione S-transferases (GSTs, EC 2.5.1.18) are multifunctional and multigene products. They are versatile enzymes and participate in the nucleophilic attack of the sulphur atom of glutathione on the electrophilic centers of various endogenous and xenobiotic compounds. Out of the five, , and , major classes of GSTs, GST- has significance in the diagnosis of cancers as it is expressed abundantly in tumor cells. This protein is a single gene product, coded by seven exons, that is having 24 kDa mass and pI value of 7.0. Four upstream elements such as two enhancers, and one of each of AP-1 site and GC box regulate gene. During chemical carcinogenesis because of jun/fos oncogenes (AP-1) regulatory elements, specifically GST- is expressed in liver. Therefore this gene product could be used as marker protein for the detection of chemical toxicity and carcinogenesis.     

A two-dimensional gas of interacting electric dipoles with hard cores: Van der Waals isotherms and their possible application in lipid phase transitions

We calculate the thermodynamic properties of a two-dimensional fluid of hard disks with embedded dipoles. Our attention is centered on the isotherms in the neighborhood of the critical point. Evaluating the canonical partition function by the "factor cluster expansion", we exhibit the Van der Waals loops obtained considering the exact two-body clusters and the "hard core" contribution of the three-body clusters. The Van der Waals isotherms can be scaled as universal functions of the parameter =p²/4r ₀ ³ kT, where p, r₀, , are the dipole moment, hard core radius, and permittivity which characterize the interaction. The model is applied to the lipid phase transition found in natural and synthetic membranes. The typical critical parameters (T_c300K, _C50 dyne/cm) reflect a physically reasonable value for the dipole moment of a polar head group of a lipid but a much-too-small value for the hard core radius.     

Effects of Osmotic Drought Stress Induced by a Combination of NaCl and Polyethylene Glycol on Leaf Water Status,Photosynthetic Gas Exchange,and Water Use Efficiency of Pistacia khinjuk and P. mutica

Leaf water potential, leaf osmotic potential, chlorophyll a and b contents, stomatal conductance, net photosynthetic rate, and water use efficiency were determined in two pistachio species (Pistacia khinjuk L. and P. mutica L.) grown under osmotic drought stress induced by a combination of NaCl and polyethylene glycol 6000. A decrease in values for all mentioned variables was observed as the osmotic potential of the nutrient solution (_s) decreased. The osmotic adjustment () of the species increased by decreasing _s. Thus P. khinjuk had a higher osmotic drought stress tolerance than P. mutica.     

Correlation between loss of turgor and accumulation of abscisic acid in detached leaves

Mature leaves of Phaseolus vulgaris L. (red kidney bean), Xanthium strumarium L. (cocklebur), and Gossypium hirsutum L. (cotton) were used to study accumulation of abscisic acid (ABA) during water stress. The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water. The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not. At least 8 h were required for a new steady-state level of ABA to be established. The samples from any one leaf covered a range of known water potentials (), osmotic pressures (), and turgor pressures (p). The and p values were calculated from pressure-volume curves, using a pressure bomb to measure the water potentials. Decreasing water potential had little effect on ABA levels in leaves at high turgor. Sensitivity of the production of ABA to changes in progressively increased as turgor approached zero. At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples. Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples. ABA levels rose steeply at different water potentials for different leaves, according to the at which turgor became zero. These differences were caused by the different osmotic pressures of the leaves that were used; must cqual - for turgor to be zero. Leaves vary in , not only among species, but also between plants of one and the same species depending on the growing conditions. A difference of 6 bars (calculated at =0) was found between the osmotic pressures of leaves from two groups of G. hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed. When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants. On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves.Abbreviations ABA abscisic acid - me-ABA abscisic-acid methyl ester - leaf water potential - osmotic pressure - p volumeaveraged turgor - volumetric modulus of elasticity     

Identification of a glutathione-S-transferase effector domain for inhibition of jun kinase, by molecular dynamics

We have recently found that the glutathione-S-transferase -isozyme (GST-), a cellular detoxification enzyme, potently and selectively inhibits activation of jun protein by its upstream kinase, jun kinase (JNK). This newly identified regulatory activity of GST- is strongly inhibited by a group of agents that inhibit its enzymatic activity. Since loss of enzymatic activity in general does not correlate with loss of regulatory activity, it is likely that inhibitor binding induces changes in the structure of one or more domains of GST that block its interaction with JNK. To identify regions of GST that change conformation on the binding of inhibitors, we have performed molecular dynamics calculations on GST- to compute its average structure in the presence and absence of the inhibitor, glutathione sulfonate. Superposition of the two average structures reveals that several regions change local structure depending upon whether the inhibitor is bound or not bound. Two of these regions, residues 36–50 and 194–201, are highly exposed. We have synthesized peptides corresponding to these two segments and find that the 194–201 sequence strongly inhibits the ability of GST- to block the in vitro phosphorylation of jun by JNK. These results suggest that this region of GST- is critical to its functioning as a newly discovered regulator of signal transduction.